DI-HYBRID CROSSES 189 



The three individuals of appearance aB include the 

 following types : 



One pure, aBaB, breeding true to the (new) 

 smooth white type. 



Two aBab, giving both smooth white and 

 wrinkled white. 



The remaining individual is db in appearance and 

 dbab in constitution, and breeds true to the wrinkled 

 white type. 



The expected behaviour of all these different types 

 can be followed out by the aid of suitable breeding 

 experiments, and not only has this been done in the 

 case of the cross which we have been considering, but 

 precisely similar phenomena have been shown to be 

 taking place for a large number of other characters in 

 many different species of plants and in a good many 

 animals as well. 



We are now in a position to state the important 

 proposition known as Mendel's law, which is to the 

 following effect : 



The gametes of a heterozygote bear the pure parental 

 allelomorphs completely separated from one another, 

 and the numerical distribution of the separate allelo- 

 morphs in the gametes is such that all possible com- 

 binations of them are present in approximately equal 

 numbers. (Note that it is impossible for more than 

 one member of the same pair of allelomorphs to occur 

 together in the same gamete.) 



This is the essence of the great discovery made by 

 Gregor Mendel, Abbot of Brunn, and published by 



